Unique electronic and optical properties of stacking-modulated bilayer graphene under external magnetic fields.

This study delves into the magneto-electronic and magneto-optical properties of stacking-modulated bilayer graphene. By manipulating domain walls (DWs) across AB-BA domains periodically, we unveil oscillatory Landau subbands and the associated optical excitations. The DWs act as periodic potentials, yielding fascinating 1D spectral features. Our exploration reveals 1D phenomena localized to Bernal stacking, DW regions, and stacking boundaries, highlighting the intriguing formation of Landau state quantization influenced by the commensuration between the magnetic length and the system. The stable quantized localization within different regions leads to the emergence of unconventional quantized subbands. This study provides valuable insights into the essential properties of stacking-modulated bilayer graphene.

Polaritonic and excitonic semiclassical time crystals based on TMDC strips in an external periodic potential.

We investigated the dynamics of Bose-Einstein condensates (BECs) under an external periodic potential. We consider two such systems, the first being made of exciton-polaritons in a nanoribbon of transition metal dichalcogenides (TMDCs), such as MoSe[Formula: see text], embedded in a microcavity with a spatial curvature, which serves as the source of the external periodic potential. The second, made of bare excitons in a nanoribbon of twisted TMDC bilayer, which naturally creates a periodic Moiré potential that can be controlled by the twist angle. We proved that such systems behave as semiclassical time crystals (TCs). This was demonstrated by the fact that the calculated BEC spatial density profile shows a non-trivial long-range two-point correlator that oscillates in time. These BECs density profiles were calculated by solving the quantum Lindblad master equations for the density matrix within the mean-field approximation. We then go beyond the usual mean-field approach by adding a stochastic term to the master equation which corresponds to quantum corrections. We show that the TC phase is still present.

Anisotropic optical conductivities of model topological nodal-line semimetals.

With the use of simple models, we investigated the optical conductivity of a nodal-line semimetal (NLSM) whose crossing of the conduction and valence bands near the origin (Opoint) in the (kx,ky) plane of a small cubic region can be adjusted by a parameterα. The Hamiltonian of the NLSM is based on thek⋅pmodel for the low-lying energy bands. Whenα = 0, these bands touch each other along a continuous closed loop but the opening of a band gap corresponding to finite values ofαand the varying of the carrier concentration can be adjusted. This provides a tunable semiconductor gap, around theOpoint and the valence and conduction bands can meet at a pair of points within the small cubic region inkspace. The optical conductivity of such a NLSM is calculated using the Kubo formula with emphasis on the optical spectral weight redistribution, deduced from appropriate Green's functions, brought about by changes in gap and chemical potential due to modifyingα. We derived closed-form semi-analytic expressions for the longitudinal components of the optical conductivity for these model systems of NLSM and compare results for chosenαand chemical potential. We also present results for the heat capacity when the system is in thermal equilibrium for various chosenαand chemical potential.

Magnetoplasmons in magic-angle twisted bilayer graphene.

The magic-angle twisted bilayer graphene (MATBLG) has been demonstrated to exhibit exotic physical properties due to the special flat bands. However, exploiting the engineering of such properties by external fields is still in it infancy. Here we show that MATBLG under an external magnetic field presents a distinctive magnetoplasmon dispersion, which can be significantly modified by transferred momentum and charge doping. Along a wide range of transferred momentum, there exist special pronounced single magnetoplasmon and horizontal single-particle excitation modes near charge neutrality. We provide an insightful discussion of such unique features based on the electronic excitation of Landau levels quantized from the flat bands and Landau damping. Additionally, charge doping leads to peculiar multiple strong-weight magnetoplasmons. These characteristics make MATBLG a favorable candidate for plasmonic devices and technology applications. .

Fundamental properties of alkali-intercalated bilayer graphene nanoribbons.

Along with the inherent remarkable properties of graphene, adatom-intercalated graphene-related systems are expected to exhibit tunable electronic properties. The metal-based atoms could facilitate multi-orbital hybridizations with the out-of-plane π-bondings on the carbon honeycomb lattice, which dominate the fundamental properties of chemisorption systems. In this work, using first-principles calculations, the feature-rich properties of alkali-metal intercalated graphene nanoribbons (GNRs) are investigated, including edge passivation, stacking configurations, intercalation sites, stability, charge density distribution, magnetic configuration, and electronic properties. There exists a transformation from finite gap semiconducting to metallic behaviors, indicating enhanced electrical conductivity. It arises from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement, edge structure, and stacking order. Moreover, the decoration of edge structures with hydrogen and oxygen atoms is considered to provide more information about the stability and magnetization due to the ribbons' effect. These findings will be helpful for experimental fabrication and measurements for further investigation of GNR-based materials.

Floquet engineering of tilted and gapped Dirac bandstructure in 1T[Formula: see text]-MoS[Formula: see text].

We have developed a rigorous theoretical formalism for Floquet engineering, investigating, and subsequently tailoring most crucial electronic properties of 1T[Formula: see text]-MoS[Formula: see text] by applying an external high-frequency dressing field within the off-resonance regime. It was recently demonstrated that monolayer semiconducting 1T[Formula: see text]-MoS[Formula: see text] exhibits tunable and gapped spin- and valley-polarized tilted Dirac bands. The electron-photon dressed states depend strongly on the polarization of the applied irradiation and reflect a full complexity of the low-energy Hamiltonian for non-irradiated material. We have calculated and analyzed the properties of the electron dressed states corresponding to linear and circular polarization of a dressing field by focusing on their symmetry, anisotropy, tilting, direct and indirect band gaps. Circularly polarized dressing field is known for transition into a new electronic state with broken time-reversal symmetry and a non-zero Chern number, and therefore, the combination of these topologically non-trivial phases and transitions between them could reveal some truly unique and previously unknown phenomena and applications. We have also computed and discussed the density of states for various types of 1T[Formula: see text]-MoS[Formula: see text] materials and its modification in the presence of a dressing field.

Plasmon Damping Rates in Coulomb-Coupled 2D Layers in a Heterostructure.

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations, which demonstrate how the Coulomb interaction renormalizes the plasmon frequencies. Most importantly, we have conducted a thorough investigation of how the decay rates of the plasmons in these heterostructures are affected by the Coulomb coupling between different types of two-dimensional materials whose separations could be varied. A novel effect of nullification of the silicene band gap is noticed when graphene is introduced into the system. To utilize these effects for experimental and industrial purposes, graphical results for the different parameters are presented.

Superfluidity of Dipolar Excitons in a Double Layer of α - T3 with a Mass Term.

We predict Bose-Einstein condensation and superfluidity of dipolar excitons, formed by electron-hole pairs in spatially separated gapped hexagonal α-T3 (GHAT3) layers. In the α-T3 model, the AB-honeycomb lattice structure is supplemented with C atoms located at the centers of the hexagons in the lattice. We considered the α-T3 model in the presence of a mass term which opens a gap in the energy-dispersive spectrum. The gap opening mass term, caused by a weak magnetic field, plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system. The band structure of GHAT3 monolayers leads to the formation of two distinct types of excitons in the GHAT3 double layer. We consider two types of dipolar excitons in double-layer GHAT3: (a) "A excitons", which are bound states of electrons in the conduction band (CB) and holes in the intermediate band (IB), and (b) "B excitons", which are bound states of electrons in the CB and holes in the valence band (VB). The binding energy of A and B dipolar excitons is calculated. For a two-component weakly interacting Bose gas of dipolar excitons in a GHAT3 double layer, we obtain the energy dispersion of collective excitations, the sound velocity, the superfluid density, and the mean-field critical temperature Tc for superfluidity.

Role Played by Edge-Defects in the Optical Properties of Armchair Graphene Nanoribbons.

We explore the implementation of specific optical properties of armchair graphene nanoribbons (AGNRs) through edge-defect manipulation. This technique employs the tight-binding model in conjunction with the calculated absorption spectral function. Modification of the edge states gives rise to the diverse electronic structures with striking changes in the band gap and special flat bands at low energy. The optical-absorption spectra exhibit unique excitation peaks, and they strongly depend on the type and period of the edge extension. Remarkably, there exist the unusual transition channels associated with the flat bands for selected edge-modified systems. We discovered the special rule governing how the edge-defect influences the electronic and optical properties in AGNRs. Our theoretical prediction demonstrates an efficient way to manipulate the optical properties of AGNRs. This might be of importance in the search for suitable materials designed to have possible technology applications in nano-optical, plasmonic and optoelectronic devices.

Tailoring plasmon excitations in [Formula: see text] armchair nanoribbons.

We have calculated and investigated the electronic states, dynamical polarization function and the plasmon excitations for [Formula: see text] nanoribbons with armchair-edge termination. The obtained plasmon dispersions are found to depend significantly on the number of atomic rows across the ribbon and the energy gap which is also determined by the nanoribbon geometry. The bandgap appears to have the strongest effect on both the plasmon dispersions and their Landau damping. We have determined the conditions when relative hopping parameter [Formula: see text] of an [Formula: see text] lattice has a strong effect on the plasmons which makes our material distinguished from graphene nanoribbons. Our results for the electronic and collective properties of [Formula: see text] nanoribbons are expected to find numerous applications in the development of the next-generation electronic, nano-optical and plasmonic devices.

Engineering plasmon modes and their loss in armchair graphene nanoribbons by selected edge-extended defects.

The effect of edge modification of armchair graphene nanoribbons (AGNRs) on the collective excitations are theoretically investigated. The tight-binding method is employed in conjunction with the dielectric function. Unconventional plasmon modes and their association with the flat bands of the specially designed AGNRs are thoroughly studied. We demonstrate the robust relationship between the novel collective excitations and both the type and period of the edge modification. Additionally, we reveal that the main features displayed in the (momentum, frequency)-phase diagrams for both single-particle and collective excitations of AGNRs can be efficiently tuned by edge-extended defects. Our obtained plasmon modes are found to be analogous to magnetoplasmons associated with collective excitations of Landau-quantized electrons. This work provides a unique way to engineer discrete magnetoplasmon-like modes of AGNRs in the absence of magnetic field.

Coherent-scatterer enhancement and Klein-tunneling suppression by potential barriers in gapped graphene with chirality-time-reversal symmetry.

We have utilized the finite-difference approach to explore electron-tunneling properties in gapped graphene through various electrostatic-potential barriers ranging from Gaussian to a triangular envelope function in comparison with a square potential barrier. The transmission coefficient is calculated numerically for each case and applied to the corresponding tunneling conductance. It is well known that Klein tunneling in graphene will be greatly reduced in gapped graphene. Our results further demonstrate that such a decrease of transmission can be significantly enhanced for spatially-modulated potential barriers. Moreover, we investigate the effect from a bias field applied to those barrier profiles, from which we show that it enables the control of electron flow under normal incidence. Meanwhile, the suppression of Klein tunneling is found more severe for a non-square barrier and exhibits a strong dependence on bias-field polarity for all kinds of barriers. Finally, roles of a point impurity on electron transmission and conductance are analyzed with a sharp peak appearing in electron conductance as the impurity atom is placed in the middle of a square barrier. For narrow triangular and Gaussian barriers, however, the conductance peaks become significantly broadened, associated with an enhancement in tunneling conductance.

Temperature-Induced Plasmon Excitations for the α-T3 Lattice in Perpendicular Magnetic Field.

We have investigated the α-T3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α-T3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits. Central to the calculation of these collective modes is the dielectric function which is determined by the polarizability of the system. This latter function is generated by transition energies between subband states, as well as the overlap of their wave functions.

Screening in Graphene: Response to External Static Electric Field and an Image-Potential Problem.

We present a detailed first-principles investigation of the response of a free-standing graphene sheet to an external perpendicular static electric field E. The charge density distribution in the vicinity of the graphene monolayer that is caused by E was determined using the pseudopotential density-functional theory approach. Different geometries were considered. The centroid of this extra density induced by an external electric field was determined as zim = 1.048 Å at vanishing E, and its dependence on E has been obtained. The thus determined zim was employed to construct the hybrid one-electron potential which generates a new set of energies for the image-potential states.

Defect capturing and charging dynamics and their effects on magneto-transport of electrons in quantum wells.

The calculated defect corrections to the polarization and dielectric functions for Bloch electrons in quantum wells are presented. These results were employed to derive the first two moment equations from the Boltzmann transport theory and then applied to explore the role played by defects on the magneto-transport of Bloch electrons. Additionally, we have derived analytically the inverse momentum-relaxation time and mobility tensor for Bloch electrons by making use of the screened defect-corrected polarization function. Based on quantum-statistical theory, we have investigated the defect capture and charging dynamics by employing a parameterized physics-based model for defects to obtain defect wave functions. Both capture and relaxation rates, as well as the density for captured Bloch electrons, were calculated self-consistently as functions of temperature, doping density and chosen defect parameters. By applying the energy-balance equation, the number of occupied energy levels and the chemical potential of defects were determined, with which the transition rate for defect capturing was obtained. By applying these results, the defect energy-relaxation, capture and escape rates, and Bloch-electron chemical potential were calculated self-consistently for a non-canonical subsystem of Bloch electrons. At the same time, the energy- and momentum-relaxation rates of Bloch electrons, as well as the current suppression factor, were also investigated quantitatively. By combining all these results, the temperature dependence of the Hall and longitudinal mobilities was presented for Bloch electrons in either single- or multi-quantum wells.

Atomistic Band-Structure Computation for Investigating Coulomb Dephasing and Impurity Scattering Rates of Electrons in Graphene.

In this paper, by introducing a generalized quantum-kinetic model which is coupled self-consistently with Maxwell and Boltzmann transport equations, we elucidate the significance of using input from first-principles band-structure computations for an accurate description of ultra-fast dephasing and scattering dynamics of electrons in graphene. In particular, we start with the tight-binding model (TBM) for calculating band structures of solid covalent crystals based on localized Wannier orbital functions, where the employed hopping integrals in TBM have been parameterized for various covalent bonds. After that, the general TBM formalism has been applied to graphene to obtain both band structures and wave functions of electrons beyond the regime of effective low-energy theory. As a specific example, these calculated eigenvalues and eigen vectors have been further utilized to compute the Bloch-function form factors and intrinsic Coulomb diagonal-dephasing rates for induced optical coherence of electron-hole pairs in spectral and polarization functions, as well as the energy-relaxation time from extrinsic impurity scattering of electrons for non-equilibrium occupation in band transport.

Magnetoplasmons for the α-T3 model with filled Landau levels.

Using the $\alpha-T_3$ model, we carried out analytical and numerical calculations for the static and dynamic polarization functions in the presence of a perpendicular magnetic field. The model involves a parameter $\alpha$ which is the ratio of the hopping strength from an atom at the center of a honeycomb lattice to one of the atoms on the hexagon to the hopping strength around its rim. Our results were employed to determine the longitudinal dielectric function and the magnetoplasmon dispersion relation. The magnetic field splits the continuous valence, conduction and at energy subband into discrete Landau levels which present significant effects on the polarization function and magnetoplasmon dispersion. This includes the fact that the energies of the Landau levels are valley dependent which leads to different behaviors of the polarization function as the hopping parameter $\alpha$ (or $\phi = tan^{-1}\alpha$) is reduced continuously toward zero. This essential critical behavior of the polarization function leads to a softening of a magnetoplasmon mode. We present results for a doped layer in the integer quantum Hall regime for fixed hopping parameter $\alpha$ and various magnetic fields as well as chosen magnetic field and different $\alpha$ in the random phase approximation.

Rich essential properties of Si-doped graphene.

The diverse structural and electronic properties of the Si-adsorbed and -substituted monolayer graphene systems are studied by a complete theoretical framework under the first-principles calculations, including the adatom-diversified geometric structures, the Si- and C-dominated energy bands, the spatial charge densities, variations in the spatial charge densities and the atom- and orbital-projected density of states (DOSs). These critical physical quantities are unified together to display a distinct physical and chemical picture in the studying systems. Under the Si-adsorption and Si-substitution effects, the planar geometric structures are still remained mainly owing to the very strong C-C and Si-C bonds on the honeycomb lattices, respectively. The Si-adsorption cases can create free carriers, while the finite- or zero-gap semiconducting behaviors are revealed in various Si-substitution configurations. The developed theoretical framework can be fully generalized to other emergent layered materials. The Si-doped graphene systems might be a highly promising anode material in the lithium-ion battery owing to its rich potential properties.

Fundamental Properties of Metal-Adsorbed Silicene: A DFT Study.

Sodium, magnesium, and aluminum adatoms, which possess one, two, and three valence electrons, respectively, in terms of 3s, 3s2, and (3s2, 3p) orbitals, are very suitable for helping us understand adsorption-induced diverse phenomena. In this work, the revealing properties of metal (Na/Mg/Al)-adsorbed graphene systems are investigated by means of the first-principles method. The single- and double-sided chemisorption cases, the various adatom concentrations, the hollow/top/valley/bridge sites, and the buckled structures are taken into account. The hollow and valley adsorptions that correspond to the Na/Mg and Al cases, respectively, create extremely nonuniform environments. This leads to diverse orbital hybridizations in Na/Mg/Al-Si bonds, as indicated by the Na/Mg/Al-dominated bands, as well as the spatial charge density distributions and the orbital-projected density of states (DOS). Out of three types of metal-adatom adsorptions, the Al-adsorption configurations produce the strongest chemical modifications. The ferromagnetic configurations have been shown to survive only in specific Mg and Al adsorptions, but not in the Na cases. The presented theoretical predictions could be verified experimentally, and potential applications are discussed. Additionally, important similarities and differences with graphene-related systems are examined.

Many-body effects and optical properties of single and double layer α-[Formula: see text] lattices.

An extensive analytical and numerical investigation has been carried out to examine the role played by many-body effects on various α-[Formula: see text] materials under an off-resonance optical dressing field. Additionally, we explore its dependence on the hopping parameter α as well as the electron-light coupling strength λ 0. The obtained dressed states due to mutual interaction between Dirac electrons and incident light are shown to demonstrate rather different electronic and optical properties in comparison with those in the absence of incident light. Specifically, various collective transport and optical properties of these electron dressed states are discussed in detail and compared for both single- and double layer α-[Formula: see text] lattices. All of these novel properties are due to the presence of a middle flat band and the interband transitions between it and an upper conduction band. Also, coupled plasmon dispersions for interacting double layer α-[Formula: see text] lattices are calculated, revealing a lower acoustic-like plasmon branch with tunable group velocity determined by both the layer separation and Fermi energy of each layer. Finally, a many-body theory is presented within the random-phase approximation for calculating the optical absorbance of doped multi-layered α-[Formula: see text] lattices in a linearly-polarized light field. We anticipate that the discoveries reported here could impact the design of the next-generation nano-optical and nano-plasmonic devices.